Proving Grounds was broken in MoP. Different classes/specs scale differently with gear, that's why WoD PG is way better than previous version. Have you tried to do MoP PG as fire mage or feral druid or rog? That was disaster... all you had to do was to chose 'correct' class and you were breezing through it, while other classes were out of luck. Now you can play whichever class you want, just need to prepare for it accordingly.
I also believe that it's gonna be even better in Legion, since they are trying to play a bit with stat scaling. So some classes (eg. fire mages) should be viable at start as well, and not only classes that scale poorly with gear. We just need to wait and see if that will work out well.
OT: Everyone saying that ilvl is everything, but I have to disagree. I think correct stats are most important, and correct ilvl for your spec is 2nd thing you should take into consideration. Also don't forget to enchant all your low lvl gear (if you gonna go for low ilvl that is, some classes scale better with low ilvl and some better with high), because that's gonna boost your perform a lot (keep in mind that end game enchants give bigger boost to lower ilvl scaling). But choosing correct stats will be like difference between day and night, so I would start there.
Apart from that, just experiment with talents and see what works best for you. You can also watch some videos, but that could mean you would need to adjust your playstyle and waste some time :/ But it's only Gold so IMO changing your gear to fit holy and going for lower ilvl + enchants (I believe for Gold even cheapest enchants will work great) will make you pass it without problems.